auxdisplay: line-display: fix OOB read on zero-length message_store()

auxdisplay: line-display: fix OOB read on zero-length message_store()

linedisp_display() unconditionally reads msg[count - 1] before
checking whether count is zero, so a write of zero bytes to the
message sysfs attribute hits msg[-1]:

	write(fd, "", 0);

	-> message_store(..., buf, count=0)
	   -> linedisp_display(linedisp, buf, count=0)
	      -> msg[count - 1] == '\n'  ; OOB read

The kernfs write buffer for that store is a 1-byte allocation
(kernfs_fop_write_iter() does kmalloc(len + 1) with len == 0),
so msg[-1] is a 1-byte read before the slab object. On a
KASAN-enabled kernel this trips an out-of-bounds report and
panics; on stock kernels it silently reads adjacent slab data
and, if that byte happens to be '\n', the following count--
wraps ssize_t 0 to -1 and is then passed to kmemdup_nul().

linedisp_display() is reached from the message_store() sysfs
callback (drivers/auxdisplay/line-display.c message attribute,
mode 0644) and from the in-tree initial-message setup with
count == -1, so the OOB path is only userspace-triggerable via
zero-byte writes; vfs_write() does not short-circuit on
count == 0 and kernfs_fop_write_iter() dispatches the store
callback regardless.

Guard the trailing-newline trim with a count check. The
existing if (!count) block then takes the clear-display path
unchanged.

Affects every auxdisplay driver that registers via
linedisp_register() / linedisp_attach(): ht16k33, max6959,
img-ascii-lcd, seg-led-gpio.

Fixes: 7e76aece6f03 ("auxdisplay: Extract character line display core support")
Signed-off-by: Stepan Ionichev <sozdayvek@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>